Mmp14 expression peaks during puberty, and is highly elevated at the invading front of mammary gland (MG) end buds. (A) Quantitative RT-PCR analysis of Mmp14 expression during development of the mouse MG from virgin (V; 2-3, 6-8 and 9-14 weeks after birth), early pregnancy (Early-preg; day 4), mid-pregnancy (Mid-preg; days 8-12), late pregnancy (Late-Preg; days 16-18) and lactating (days 1-10) mice, normalized to 18S rRNA. Data are mean±s.e.m. **P<0.01 when compared with V (2-3w). (B) Mmp14 promoter activity in MGs from Mmp14 (+/lacZ) mice. Images are of glands of 5-week-old mice. β-Gal staining of a whole-mount MG isolated from a virgin transgenic heterozygote mouse bearing the lacZ gene under the control of the endogenous Mmp14 promoter (Yana et al., 2007) indicates that Mmp14 promoter activity is high in mammary epithelial cells. (i) β-Gal-stained MG from Mmp14 (+/+) mouse as negative control. (ii) β-gal stained MG from Mmp14 (+/lacZ) mouse. Scale bar: 6.25 mm. (iii) β-Gal-stained mammary end buds in Mmp14 (+/lacZ) mouse showed intense promoter activity at the tip of the bud. (iv) β-Gal and Eosin stain of a MG tissue section from a 5-week-old Mmp14 (+/lacZ) transgenic mouse. Scale bars: 200 μm.